1. Field of the Invention
The present invention relates to analog-digital converters using .DELTA..SIGMA. modulation.
2. Prior Art
Conventionally, there are provided one-bit analog-digital converters which use .DELTA..SIGMA. modulation system. FIG. 1 shows an example of a .DELTA..SIGMA. analog-digital converter conventionally known. In FIG. 1, a .DELTA..SIGMA. modulator 31 converts an analog input into serial-bit strings. This .DELTA..SIGMA. modulator 31 consists of a switched-capacitor integrator 32, a one-bit quantizer 33 and a feedback circuit 34. Herein, the one-bit quantizer 33, which is configured using a clocked comparator, is provided to quantize an output of the switched-capacitor integrator 32; and an output of the one-bit quantizer 33 is subjected to one-sample delay in the feedback circuit 34 based on reference voltage, wherein either positive reference voltage V.sub.REF+ or negative reference voltage V.sub.REF- is selected as the reference voltage in response to polarity of the output of the one-bit quantizer 33. Serial-bit strings, which are produced by the .DELTA..SIGMA. modulator 31, are inputted into a digital filter 35. The digital filter 35 extracts low-frequency components, corresponding to the analog input, from the serial-bit strings, so the low-frequency components extracted are converted into digital data of certain number of bits.
The above-mentioned analog-digital converter employs a so-called "scaling system" (disclosed by U.S. Pat. No. 4,851,841) in order to reduce noise in a digital output. According to the scaling system, gain of the .DELTA..SIGMA. modulator 31 is fixed at `1/A`, while scaling gain `A` is provided for the digital filter 35 by using impulse-response coefficients. Such a system is made under consideration of a fact that the analog-digital converter of FIG. 1 has certain noise characteristic, as shown by FIG. 2, with respect to analog-input level.
In the analog-digital converter of FIG. 1, as shown by FIG. 2, as the analog input becomes closer to full scale in analog-input level, i.e., "clip level" of the .DELTA..SIGMA. modulator 31, level of noise in the digital output becomes larger, wherein the clip level of the .DELTA..SIGMA. modulator 31 is set at either V.sub.REF+ or V.sub.REF-. Therefore, the gain of the .DELTA..SIGMA. modulator 31 is suppressed in such a way that maximum value in the analog input coincides with the reference voltage multiplied by `1/A` (e.g., `0.8`). In contrast to suppression of the gain of the .DELTA..SIGMA. modulator 31, scaling gain `A` is applied to the digital filter 35 which follows the .DELTA..SIGMA. modulator 31. By employing such a scaling system, it is possible to effectively reduce the noise.
Meanwhile, if the analog input contains DC offset component, "calibration" is required to remove the DC offset component at last. In order to do so, the digital filter 35 is followed by a high-pass filter, for example. However, when providing the high-pass filter which follows the digital filter 35 under a condition where a certain gain is applied to the digital filter by employing the aforementioned scaling system, some problem occurs due to clipping for high-level signals.
Now, we will explain about the aforementioned problem with reference to FIGS. 3A and 3B. Let us think about it by using an analog-input signal, whose amplitude is relatively large and which contains a DC offset `.DELTA.V`, as shown by FIG. 3A, for example. Herein, `.+-.V1` indicate clip levels of the digital filter 35; and `.+-.V2` indicates clip levels of the .DELTA..SIGMA. modulator 31. The above-mentioned clip levels .+-.V1 and .+-.V2 are shown relatively with respect to altering level of the analog-input signal, so relationship between clip levels, which are actually set for the analog-digital converter of FIG. 1, do not necessarily coincide with relationship between .+-.V1 and .+-.V2 shown by FIG. 3A. Thanks to the aforementioned scaling system, maximum amplitude of the analog-input signal can be suppressed less than the clip level of the .DELTA..SIGMA. modulator 31. On the other hand, the scaling gain A is applied to the digital filter 35, so the clip level becomes lower. If the DC offset .DELTA. exists in a positive side of the analog-input signal shown by FIG. 3A, there may occur a situation where waveform in the positive side of the analog-input signal (i.e., a positive part of the output of the high-pass filter) is clipped at a certain level as shown by FIG. 3B. In that situation, the DC offset can be removed, however, a clipped state in positive side of the waveform may remain in an output of the high-pass filter. Such a clipped state may cause to occur deformation corresponding to overflow of data.
As described above, in order to reduce the noise, the scaling system is applied to the analog-digital converter, based on the .DELTA..SIGMA. modulation, so that gain of the modulator is limited while a certain gain is applied to the digital filter which follows the modulator. In this case, if DC offset is contained by the analog-input signal, a clipped state may occur for a large amplitude of the analog-input signal. So, there is a problem that un-desired effect due to such a clipped state cannot be eliminated even if a high-pass filter is provided to remove the DC offset.